A surge absorbing element has a function of causing a surge current to flow to protect a subsequent-stage circuit when a voltage equal to or higher than a predetermined value is applied. The surge absorbing element generally has a structure in which a pair of electrodes is attached to both ends of a varistor substrate made of ZnO or the like, respectively, external leads are drawn from the respective electrodes, and the varistor substrate and the electrodes are covered by an exterior member.
Due to a current flowing in the varistor substrate, an operation start voltage lowers. That is, a flow of a current deteriorates the function of the surge absorbing element and gradually brings the varistor substrate closer to a short-circuit state. Accordingly, when an excessive surge voltage is applied to the varistor substrate many times and the varistor substrate is further deteriorated, the excessive surge voltage finally causes a short-circuit failure.
For example, Patent Literature 1 describes a metal oxide varistor with bimetal, which has such a function that bimetal is incorporated in a metal oxide varistor (a surge absorbing element) for absorbing a surge voltage to be used to protect an electronic component.